FIG. 15 is a simplified diagram showing an exemplary conventional H-pattern contact solar cell 40H that converts sunlight into electricity by the photovoltaic effect. Solar cell 40H is formed on a semiconductor (e.g., poly-crystalline or mono-crystalline silicon) substrate 41H that is processed using known techniques to include an n-type doped upper region 41U and a p-type doped lower region 41L such that a pn-junction is formed near the center of substrate 41H. Disposed on a frontside surface 42H of semiconductor substrate 41H are a series of parallel metal gridlines (fingers) 44H (shown in end view) that are electrically connected to n-type region 41U. A substantially solid conductive layer 46 is formed on a backside surface 43 of substrate 41H, and is electrically connected to p-type region 41L. An antireflection coating 47 is typically formed over upper surface 42 of substrate 41H. Solar cell 40H generates electricity when a photon from sunlight beams L1 pass through upper surface 42 into substrate 41H and hit a semiconductor material atom with an energy greater than the semiconductor band gap, which excites an electron (“−”) in the valence band to the conduction band, allowing the electron and an associated hole (“+”) to flow within substrate 41H. The pn-junction separating n-type region 41U and p-type region 41L serves to prevent recombination of the excited electrons with the holes, thereby generating a potential difference that can be applied to a load by way of gridlines 44H and conductive layer 46, as indicated in FIG. 15.
FIGS. 16(A) and 16(B) are perspective views showing the frontside contact patterns of solar cells 40P and 40M, which are respectively formed on a square (or rectangular) poly-crystalline silicon (“poly-silicon”) substrate 41P and an octagonal (or other non-square shape) mono-crystalline silicon (mono-silicon) substrate 41M. Both solar cells 40P and 40M have frontside contact patterns consisting of an array of parallel narrow gridlines 44P/44M and one or more wider collection lines (bus bars) 45 that extend perpendicular to gridlines 44P/44M, both gridlines 44P/44M and bus bars 45 being disposed on upper surfaces 42 of substrates 41P and 41M, respectively. In both cases gridlines 44P/44M collect electrons (current) from substrates 41P/41M as described above, and bus bars 45 gather current from gridlines 44P/44M. In a photovoltaic module, bus bars 45 become the points to which metal ribbons (not shown) are attached, typically by soldering, with the ribbon being used to electrically connect one cell to another in a solar panel (i.e., an array of solar cells 40P/40M that are arranged on a common platform that are wired in series or parallel). With both types of solar cells 40P and 40M, the backside contact pattern (not shown) consists of a substantially continuous back surface field (BSF) metallization layer and multiple (e.g., three) spaced apart solder pad metallization structures that serve in a manner similar to bus bars 45 and are also connected to an associated metal ribbon (not shown) used to electrically connect one cell to another.
Those skilled in the art understand that solar cell 40P formed on poly-silicon substrate 41P is typically less expensive to produce than solar cell 40M formed on mono-silicon substrate 41M, but that solar cell 40M is more efficient at converting sunlight into electricity than solar cell 40P, thereby offsetting some of the higher manufacturing costs. Poly-silicon substrates 41P are less expensive to produce than mono-crystalline substrates 41M because the process to produce poly-silicon wafers is generally simpler and thus cheaper than mono-crystalline wafers. Typically, poly-crystalline wafers are formed as square ingots using a cast method in which molten silicon is poured into a cast and then cooled relatively quickly, and then the square ingot is cut into wafers. However, poly-silicon wafers are characterized by an imperfect surface due to the multitude of crystal grain boundaries, which impedes the transmission of sunlight into the cell, which reduces solar energy absorption and results in lower solar cell efficiency (i.e., less electricity per unit area). That is, to produce the same wattage, poly-silicon cells would need to have a larger surface area than their mono-crystalline equivalent, which is important when limited array space is available. In contrast, mono-crystalline substrates (wafers) are grown from a single crystal to form cylindrical ingots using a relatively slow (long) cooling process. The higher production costs associated with solar cells 40M are also partially attributed to the higher cost of producing mono-crystalline wafers 41M, which involves cutting the cylindrical ingot into circular disc-shaped wafers, and then cutting the circular wafers into ‘pseudo’ square (polygonal) shapes with uniform surfaces (e.g., the octagonal shape shown in FIG. 16(B)) that are efficient for assembly onto into a panel. Note that forming these ‘pseudo’ square substrates involves cutting away peripheral sections of the circular wafer, which creates a substantial waste of silicon. However, because mono-silicon substrate 41M includes a single-crystal structure, its sunlight transmission is superior to that of poly-silicon substrate 41P, which contributes to its higher operating efficiency.
Conventional methods for producing H-pattern solar cells include screen-printing and micro-extrusion. Screen-printing techniques were first used in the large scale production of solar cells, but has a drawback in that it requires physical contact with the semiconductor substrate, resulting in relatively low production yields. Micro-extrusion methods were developed more recently in order to meet the demand for low cost large-area semiconductors, and include extruding a conductive “gridline” material onto the surface of a semiconductor substrate using a micro-extrusion printhead.
Due to a market bias toward lower cost solar cells, conventional mass-production micro-extrusion systems and printheads are currently optimized to extrude a conductive paste (containing frit along with the conductive material) onto square/rectangular poly-silicon substrates 41P in accordance with the method illustrated in FIG. 17. FIG. 17 is simplified top view depicting the currently used micro-extrusion method for printing gridlines 44P onto frontside surface 42 of poly-silicon substrate 41P during the production of solar cell 40P (which is shown in a completed form in FIG. 16(A)). Poly-silicon substrate 41P is positioned below and moved in a process (Y-axis) direction relative to conventional micro-extrusion printhead 100-PA while gridline material is extruded from multiple nozzle outlet orifices 69-PA that are aligned in the cross-process (X-axis) direction, causing the extruded gridline material to form parallel gridline structures 44P on substrate 41P. The extrusion (gridline printing) process is started, e.g., by way of opening a valve feeding gridline material into printhead 100-PA, when nozzle outlet orifices 69-PA are positioned a predetermined distance from front edge 41P-F of substrate 41P such that leading edges of gridlines 44 are separated from front edge 41P-F by a predetermined gap distance S. Similarly, the gridline printing process is terminated to provide a space between the lagging ends of gridlines 44P and back edge 41P-B of substrate 41P. This gap is provided between the front/rear ends of gridlines 44P and the front/rear edges of substrate 41P in order to prevent a possible short-circuit between gridlines 44P and conductors (not shown) that are formed on the backside surface of substrate 41P. In comparison to screen printing techniques, the extrusion of dopant material onto substrate 41P provides superior control of the feature resolution of the conductive regions, and facilitates deposition without contacting substrate 41P, thereby avoiding wafer breakage (i.e., increasing production yields). Such fabrication techniques are disclosed, for example, in U.S. Patent Application No. 20080138456, which is incorporated herein by reference in its entirety.
Another problem faced by mono-crystalline-based solar cells is that current solar cell extrusion printing equipment that is optimized for poly-silicon-based solar cells cannot be used in an efficient manner to make octagonal (“pseudo-square”) mono-crystalline-based solar cells. This problem is illustrated in FIG. 18, which depicts the use of conventional printhead 100-PA to generate gridlines 44P on an octagonal mono-crystalline silicon substrate 41M. That is, conventional printhead 100-PA and the associated micro-extrusion system typically are incapable of individually controlling extrusion material passed though nozzle outlet orifices 69-PA, whereby gridlines 44P disposed along the side edges of substrate 41M extend over chamfered edges 41M-C1 and 41M-C3 (as indicated by the dashed-line circles in FIG. 18), thereby probably producing a short-circuit between gridlines 44P and conductors (not shown) that are formed on the backside surface of substrate 41M. One possible solution to this problem would be to provide a mask on the corner regions, or to remove the overlapping gridline portions. This approach would allow the use of currently available equipment, but would greatly increases manufacturing costs, would potentially reduce yields by requiring substantial pre-extrusion or post-extrusion processing of the octagonal mono-silicon wafers, and would also waste gridline material. Another possible approach would be to modify the printhead to include an individual valve for each outlet orifice, and modifying the system to facilitate individually controlling the valves such that the endpoints of each extruded gridline could be formed with the desired gap distance, whether formed along an end or chamfered edge of the substrate. However, this approach would require significant (and very expensive) changes to the micro-extrusion control system, and would be difficult to implement due to the large number of valves that would be required. Moreover, adding a large number of external valves to the existing co-extrusion system is problematic due to space limitations and payload on the existing micro-extrusion systems utilized to generate solar cells on square poly-silicon substrates.
What is needed is a method for forming gridlines on octagonal mono-crystalline silicon substrates that avoids the problems mentioned above in association with the conventional gridline printing process. What is also needed is a micro-extrusion printhead assembly that facilitates implementation of the method with minimal required modification of existing micro-extrusion systems.